1. Field of the Invention
The present invention relates to an image heating apparatus that heats an image formed on a recording material in an image forming apparatus such as a copying machine, facsimile, or a printer.
And the present invention relates to a fixing apparatus that fixes an unfixed image formed on a recording material in an image forming apparatus such as a copying machine, facsimile, or a printer.
More specifically, the present invention relates to an image heating apparatus including multiple image heating means each having an image heating member that heats an image on a recording material and a separation member that separates the recording material from the image heating member, the image heating apparatus allowing the recording material to first pass through an image heating means on an upstream side with respect to a conveying direction of the recording material and then pass through an image heating means on a downstream side.
2. Related Background Art
Image forming apparatuses such as a copying machine, which utilize an electrophotographic process includes an image heating apparatus that heats an image (unfixed developer image) formed on a sheet (recording material) through transfer or directly.
As the image heating apparatus, in recent years, image heating apparatuses including multiple image heating means are becoming popular with an increase in speed of paper passing and an improvement in image quality (see Japanese Patent Application Laid-Open No. H06-258970, for instance).
Such an image heating apparatus has multiple image heating means arranged along a conveying direction of the recording material, the multiple image heating means including image heating members that contact a recording material, on which an unfixed image is borne, and heat the image. Image heating is performed by causing the recording material to pass through the multiple image heating means in succession.
Such an image heating apparatus including multiple image heating means has such a feature that, for instance, a recording material on which an unfixed image is borne, is caused to first pass through a first image heating means on the most upstream side in a conveying direction of the recording material for image heating and fixing and then pass through a second image heating means on an downstream side for image reheating. Generally, many of such image heating apparatuses are intended to ensure a certain degree of a fixation property at the first image heating means and to achieve high image quality (high gloss) at the second image heating means. In particular, the glossiness of an image is greatly influenced by the second image heating means on the downstream side in the recording material conveying direction.
In such an image heating apparatus as well, in order to enhance separation property at the time of image fixation onto a first surface and a second surface of a recording material, it is effective to provide a separation member for each image heating member.
Incidentally, while it is possible to significantly enhance the separation property by bringing a separation claw into abutment against the image heating member, there occurs a problem in that a surface of the image heating member is damaged by the separation claw. Also, while it is possible to alleviate the damage by reciprocating the abutment separation claw, the separation claw and an image surface rub against each other, causing the surface property of a heating roller to partially change, and therefore gloss unevenness occurs. Accordingly, the provision of the separation claw has both an advantage that the separation property is improved and a disadvantage that the gloss unevenness occurs.
In addition, when the separation member of the image heating means on the upstream side and the separation member of the image heating means on the downstream side have the same construction, the following problems occur.
The image heating means on the upstream side is required to ensure sufficient separation performance because an entirely unfixed image and the image heating member contact each other and a strong adhesive force occurs between fused toner and the image heating member. On the other hand, separation property at the time when an image heated once is re-heated is high as compared with the case of heating at the image heating means on the upstream side, so it is possible to ensure a sufficient separation property even when the separation performance is lowed as compared with that at the image heating means on the upstream side.
Meanwhile, it is known that the glossiness of an image greatly depends on the surface property of the heating member that contacts the image. Consequently, unevenness of the surface property of the heating member ascribable to the separation member significantly influences unevenness of the image glossiness. Therefore, the glossiness enhancing effect is reduced when multiple image heating means are provided.
Accordingly, when the separation structure of the image heating means on the upstream side is applied to the image heating means on the downstream side, it becomes necessary to shorten the replacement cycle for the image heating member for the sake of elimination of gloss unevenness.
In particular, the problems described above become serious when stable separation property is to be obtained at the time of image heating intended for color image fixation, an increase in speed, and high productivity.